Virus and hacker attacks have shot up by 20% in the first six months of this year, according to security experts.
Some viruses try to dupe users into opening an attachment
Hackers are getting faster at finding security holes to target too, says a survey by anti-virus firm Symantec.
New ways of spreading viruses, as well as combinations of malicious code and software holes are also being used to cripple systems.
Analysts say it should be a warning to companies to be extra vigilant in protecting their systems.
The speed and use of so-called "blended threats" that combine malicious code while exploiting software holes is one of the biggest threats facing businesses this year, the report suggests.
The Slammer worm at the start of 2003 hit global systems in less than a few hours.
And the Blaster worm in the summer infected about 2,500 computers an hour, costing companies billions in damage and lost business.
With companies experiencing an average of 38 attacks a week, ensuring these vulnerabilities in systems are patched or fixed, is "critical" to their survival.
"We are seeing a slight increase in the number of vulnerabilities year on year, which is significant as customers are having a difficult time patching their systems," Symantec's Jeff Ogden told BBC News Online.
"Even more significant is that the attacks are coming quickly."
This is a concern because it means organisations have less time to respond to threats.
"The problem with blended threats is that you need multiple technologies and good management to deal with them," said Mr Ogden.
"And the threats can take many forms. It can be a piece of malicious code which attacks a vulnerability and is transported in various way, therefore you have to understand the transport method as well as what the code does."
The growing use of peer-to-peer and instant messaging (IM) programs to communicate within an organisation opens up a way of spreading malicious code, which is a significant problem for security.
Last year two vulnerabilities in peer-to-peer networking were found, but 19 have been identified so far this year.
Home users need to protect themselves too
The way peer-to-peer traffic is transported through a network is usually through the same port which is used to connect to the net.
This makes it more difficult to identify what is normal net traffic and what is malicious code.
IM users could also be unwittingly accepting malicious code in messages, for example.
Mr Ogden says it is not only a concern for businesses, but also for people at home using file-sharing services and IM.
To protect themselves, companies and home users need to use a combination of protective safeguards.
Firewall technology, regular monitoring of net traffic, intrusion prevention systems should be within a corporate infrastructure as well as appropriate anti-virus software, Mr Ogden said.
For home users it is a case of finding an appropriate piece of software that does all of this.
Recent figures from the Office of National statistics say 28% of people in the UK had had a computer virus in the last year.